FIG. 5 shows a conventional power supply for an electric discharge machine. A work gap 6 formed between a tool electrode 4 and a conductive workpiece 2 is filled with dielectric fluid, and the size of the work gap 6 is maintained at a fixed microscopic value. A D.C. power supply 8, a current limiting resistance 12 and a switching element 10 are connected in series and to the work gap 6. The switching element 10 is closed when a gate pulse signal G10 is ON, and is open when the gate pulse signal G10 is OFF. The gate pulse signal G10 has a controlled ON time and a controlled OFF time. When voltage is supplied from the D.C. power supply 8 to the work gap 6 by the switching element 10 being closed, the resistance of the dielectric fluid in the work gap 6 is reduced and electric discharge starts. When discharge current flows through the work gap 6, material of the workpiece 2 is removed. The peak value of the discharge current is determined by the current limiting resistance 12. When a set time has elapsed from detection of start of electric discharge, the gate pulse signal G10 becomes OFF and electric discharge is terminated.
U.S. Pat. No. 5,534,675 discloses the circuit of FIG. 6 containing substantially no current limiting resistance or inductance in order to conserve power. The circuit of FIG. 6 comprises a comparator 20 for comparing a voltage at the two ends of a current detecting resistor 16 with a reference voltage 18, and an AND gate 21 for supplying an output that has an ON state when the output from the comparator 20 and the gate pulse signal G10 are both ON to the switching element 10. As shown in FIG. 7A, if the gate pulse signal G10 becomes ON at time t1, a voltage of 50V is supplied from the D.C. power supply 8 to the work gap 6. FIGS. 7C and 7D respectively show a voltage Vgap and a current Igap across the work gap in FIG. 6. When electric discharge starts at time t2, the current Igap starts to flow through the work gap 6, as shown in FIG. 7D. When the current flowing in the current detecting resistor 16 exceeds the set value at time t3, the output of the comparator 20 becomes OFF and the output of the AND gate 21 also becomes OFF. As a result, the switching element 10 is opened, no current at all flows in the current detecting resistor 16, and so the outputs of the comparator 20 and the AND gate 21 both become ON again. As shown in FIG. 7B, the AND gate 21 generates high frequency pulses from time t3 until time t4 when the gate pulse signal G10 goes OFF. During the period from t3 to t4, the pulsed current Igap is maintained at a peak value determined by the reference voltage 18, while the voltage Vgap is physically maintained at 30V.
FIGS. 7E and 7F show voltage Vgap and current Igap across the work gap 6 when a higher voltage D.C. power supply 8 of 90V is used. As shown in FIG. 7F, current pulses Igap reach a set peak value a very short time .DELTA.t after time t2 when electric discharge starts. In this way, if the slope of the rising edge of the current pulse Igap is made steeper, the machining rate is increased. However, if the D.C. power supply 8 is made a higher voltage, current ripple become larger and higher in frequency. If ripple of the current pulse Igap is large, it is probable that the current pulse Igap will be terminated before the set time. High frequency ripple has the undesirable effects of increasing heat generated in the switching element 10. Also, at the time of electric discharge a voltage difference between the voltage of the D.C. power supply 8 and the voltage across the work gap 6 is expended on the switching element 10 etc., which means that a higher voltage D.C. power supply 8 outweighs the energy conservation effects of the circuit of FIG. 6.